1. Field of the Invention
This invention is concerned with apparatus and method for the monitoring of physiological parameters of a patient through the use of optical systems which do not require direct physical contact with the patient. More particularly, the method and apparatus described herein relates primarily to pulse oximetry for monitoring of pulse rate and arterial blood oxygen saturation. However, the apparatus and method of this invention are applicable to any form of optical detection of the physiological parameters in which visible or invisible light is directed into a patient and subsequently collected at a nearby location.
2. Description of the Prior Art
Pulse oximetry is generally well-known in the art. In presently-known conventional oximetry systems, light is provided from a source such as a pair of light-emitting diodes and is directed onto a pulsatile bed of tissue. The transmitted light is collected with a photodiode positioned on an opposite surface of the tissue bed in the case of transmission pulse oximetry, or on an adjacent surface of the tissue bed in the case of reflectance pulse oximetry. The light source and photodetector are generally housed in a reusable or a disposable sensor which connects to the pulse oximetry electronics and display unit. Photocurrents generated within the photodetector or light collector are detected and processed for measuring the relative amount of modulation of the transmitted red and near infrared light. It is well-known to those skilled in the art that the modulation ratio thus generated has been observed to have a high correlation with arterial oxygen saturation.
It is also well-known that accurate functioning of a pulse oximetry probe of the types presently in use requires that only light which has travelled through blood perfused tissues is collected at the sensor. Light which has not travelled through such blood perfused tissue, commonly called shunt or shunt light, causes an offset in the detected signal levels and results in an inaccurate estimate of oxygen saturation. Typical pulse oximetry sensors or probes are mounted directly on a surface of pulsatile tissue such as the finger, ear, foot or forehead of a patient. Shunt light is avoided through proper mechanical design and use of the sensor. In transmissive sensors, the emitter and detector are held against opposing surfaces of the pulsatile tissue bed so that the only light detected has necessarily travelled through the tissue. In reflectance sensors, shunts or shunt light are blocked from the sensor through the use of an opaque barrier positioned between the sensor's emitter and detector that can be pressed into or made to releasably adhere to the surface of the skin surrounding the pulsatile tissue bed.
It is thus obvious that a limitation in the use of conventional or presently-known transmissive or reflectance pulse oximetry is that the sensors all require physical contact with the patient. This contact is often inconvenient or undesirable in some patients. For example, some patients such as infants or burn victims, have fragile skin and cannot tolerate the mechanical or adhesive interaction required in the use of conventional probes. As another example, sleeping patients must generally be awakened to place the probes for a measurement of oxygen saturation. It has thus been apparent that a non-contact or remote form of pulse oximetry would be advantageous.
Prior art approaches to non-contact optical monitoring, and in particular pulse oximetry, have often related to the treatment of the eye where the advantage of not having to contact the retinal surface is obvious. Examples of prior art teachings relating to such non-contact optical monitors may be found in U.S. Pat. No. 4,305,398, issued to Sawa; U.S. Pat. No. 4,166,695, issued to Hill et al.; and U.S. Pat. No. 5,141,303, issued to Yamamoto et al. Each of these patents teaches the use of an optical system which yields reflected light information relating to the blood saturation of one or more portions of the eye. It should be noted that each of the systems taught by the prior art appear to have rather substantial mounts to assure the stability of the optical devices during measurement. Another form of non-contact optical measurement of physiological parameters is taught in U.S. Pat. No. 4,862,894, issued to Fujii. This patent teaches the use of a laser beam in combination with other optical devices for remote monitoring of the blood stream in a skin surface. This patent teaches a device which moves the laser beam across the skin causing it to image from a plurality of points which are arranged at least linearly. A light collector is provided to receive the laser beam's reflection which has been scattered by blood cells at the plurality of points on the surface of the skin.